From the principle of double-slit interference, we can know that the maximum optical path difference of interference fringes is the double-slit distance d, so there are at most d/λ bright fringes on both sides except the central bright fringe, so theoretically, at most ***2d/λ bright fringes can be seen on both sides.

In fact, it is impossible to see so much because of the brightness.

This is not an existing formula, but is derived from the basic principle of interference, because the maximum deflection angle is 90 degrees, and the difference between the two optical paths reaches the maximum value D.